Liquid Fluorine is the highest performance oxidizer and in the early 1960's it seemed in both American and Russia that a new generation of higher performance engines would emerge. However although test engines were built, fluorine was found to be just too toxic and reactive to be safely used as a propellant.

Liquid fluorine is essentially 100 per cent pure, containing only traces of oxygen, nitrogen, and hydrogen fluoride. The liquid exhibits a clear yellow color, while the gas has a pale greenish-yellow color at ambient temperature. It has a characteristic pungent halogen odor. Fluorine is the strongest oxidizing agent known. Under proper conditions, fluorine reacts with practically every element or compound except the inert gases. It is stable to shock, heat, and electric spark. It is non-flammable with air. Fluorine is highly toxic and irritating to all tissues.

Fluorine is produced from a molten mixture of HF and KF by an electrolytic procedure. The 1959 United States production of liquid fluorine was estimated at 40,000 metric tons/year. Liquid fluorine's cost, ex-works, was then $ 6.00 per kg.

In the Soviet Union Glushko founded a plant for production of fluorine propellants 23 km from Leningrad at Karlelsko. The population eventually reached 120,000, but although one engine was ready for production, this was never authorized due to the safety problems in case of a launch vehicle failure.

Development in the United States, from Ignition! by John D Clark, 1972:

Liquid fluorine work started about the same time as the liquid hydrogen work did. JPL, starting in 1947, was the pioneer. It wasn't particularly available at that time, so they made and liquefied the fluorine on the site, a feat which inspires the respect of anyone who has ever tried to make a fluorine cell work for any length of time. They burned it first with gaseous hydrogen, but by 1948 they had succeeded in firing liquid hydrogen, and were using the latter as a regenerative coolant. And by the spring of 1950 they had done the same with hydrazine. Considering the then state of the technology, their achievement was somewhat miraculous.

Bill Doyle, at North American, had also fired a small fluorine motor in 1947, but in spite of these successes, the work wasn't immediately followed up. The performance was good, but the density of liquid fluorine (believed to be 1.108 at the boiling point) was well below that of oxygen, and the military (JPL was working for the Army at that time) didn't want any part of it.

This situation was soon to change. Some of the people at Aerojet simply didn't believe Dewar's 54-year-old figure on the density of liquid fluorine, and Scott Kilner of that organization set out to measure it himself. (The Office of Naval Research put up the money.) The experimental difficulties were formidable, but he kept at it, and in July, 1951, established that the density of liquid fluorine at the boiling point was not 1.108, but rather a little more than 1.54. There was something of a sensation in the propellant community, and several agencies set out to confirm his results. Kilner was right, and the position of fluorine had to be re-examined. (ONR, a paragon among sponsors, and the most sophisticated — by a margin of several parsecs — funding agency in the business, let Kilner publish his results in the open literature in 1952, but a lot of texts and references still list the old figure. And many engineers, unfortunately, tend to believe anything that is in print.)

Several agencies immediately investigated the performance of fluorine with hydrazine and with ammonia and with mixtures of the two, and with gratifying results. Not only did they get a good per- formance, but there were no ignition problems, liquid fluorine being hypergolic with almost anything that they tried as a fuel.

Unfortunately, it was also hypergolic with just about everything else. Fluorine is not only extremely toxic; it is a super-oxidizer, and reacts, under the proper conditions with almost everything but nitrogen, the lighter of the noble gases, and things that have already been fiuorinated to the limit. And the reaction is usually violent.

It can be contained in several of the structural metals — steel, cop- per, aluminum, etc. — because it forms, immediately, a thin, inert coating of metal fluoride which prevents further attack. But if that inert layer is scrubbed off, or melted, the results can be spectacular. For instance, if the gas is allowed to flow rapidly out of an orifice or a valve, or if it touches a spot of grease or something like that, the metal is just as likely as not to ignite — and a fluorine-aluminum fire is something to see. From a distance.

But, as is usually the case, the stuff can be handled if you go about it sensibly, and if you want to fire it in a rocket, Allied Chemical Co. will be glad to ship you a trailer truck full of liquid fluorine. That trailer is a rather remarkable device in itself. The inner fluorine tank is surrounded by a jacket of liquid nitrogen, to prevent the evapora- tion and escape of any fluorine into the atmosphere. All sorts of pre- cautions—pilot trucks, police escorts, and what not—are employed when one of those trucks travels on a public road, but sometimes I've wondered what it would be like if a fluorine tank truck collided with one carrying, say, liquid propane or butane.

The development of large fluorine motors was a slow process, and sometimes a spectacular one. I saw one movie of a run made by Bell Aerosystems, during which a fluorine seal failed and the metal ignited. It looked as though the motor had two nozzles at right angles, with as much flame coming from the leak as from the nozzle. The motor was destroyed and the whole test cell burned out before the operators could shut down.

But good-sized fluorine motors have been developed and fired successfully, although none have yet flown in a space mission. Rocketdyne built Nomad, a 12,000-pound motor, burning fluorine and hydrazine, for upper stage work, and Bell developed the 35,000 pound Chariot for the third stage of Titan III. This burned fluorine and a mixture of monomethyl hydrazine, water, and hydrazine, balanced to burn to CO and HF, and to have a freezing point con- siderably below that of hydrazine. And GE has developed the 75,000 pound X-430 fluorine-hydrogen motor.

Further development of large fluorine motors was dropped in the United States by the mid-1960's. Work continued with fluorine compounds to power chemical lasers and power small antisatellite or anti-missile key vehicles.